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Vidarabine


ACCESSION NB: DB00194 (APRD00333, EXPT02753)


TYPE: small molecule


GROUP: approved


DESCRIPTION:
A nucleoside antibiotic isolated from Streptomyces antibioticus. It has some antineoplastic properties and has broad spectrum activity against DNA viruses in cell cultures and significant antiviral activity against infections caused by a variety of viruses such as the herpes viruses, the vaccinia VIRUS and varicella zoster virus. [PubChem]

VOLUME OF DISTRIBUTION: Not Available

CATEGORIES:
Antiviral Agents Antimetabolites

ABSORPTION: Systemetic absorption of vidarabine should not be expected to occur following ocular administration and swallowing lacrimal secretions.

INDICATION:
For treatment of chickenpox - varicella, herpes zoster and herpes simplex

PHARMACODYNAMICS:
Vidarabine is a synthetic purine nucleoside analogue with in vitro and in vivo inhibitory activity against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV). The inhibitory activity of Vidarabine is highly selective due to its affinity for the enzyme thymidine kinase (TK) encoded by HSV and VZV. This viral enzyme converts Vidarabine into Vidarabine monophosphate, a nucleotide analogue. The monophosphate is further converted into diphosphate by cellular guanylate kinase and into triphosphate by a number of cellular enzymes. in vitro, Vidarabine triphosphate stops replication of herpes viral DNA. When used as a substrate for viral DNA polymerase, Vidarabine triphosphate competitively inhibits dATP leading to the formation of 'faulty' DNA. This is where Vidarabine triphosphate is incorporated into the DNA strand replacing many of the adenosine bases. This results in the prevention of DNA synthesis, as phosphodiester bridges can longer to be built, destabilizing the strand.

MECHANISM OF ACTION:
Vidarabine stops replication of herpes viral DNA in 2 ways: 1) competitive inhibition of viral DNA polymerase, and consequently 2) incorporation into and termination of the growing viral DNA chain. This drug is a nucleoside analog and therefore has to be phosphorylated to be active. Vidarabine is sequentially phosphorylated by kinases to the triphosphate ara-ATP. This is the active form of vidarabine and is both an inhibitor and a substrate of viral DNA polymerase. When used as a substrate for viral DNA polymerase, ara-ATP competitively inhibits dATP leading to the formation of ‘faulty’ DNA. This is where ara-ATP is incorporated into the DNA strand replacing many of the adenosine bases. This results in the prevention of DNA synthesis, as phosphodiester bridges can longer to be built, destabilizing the strand

PROTEIN BINDING:
24-38%

METABOLISM:
In laboratory animals, vidarabine is rapidly deaminated in the gastrointestinal tract to Ara-Hx.

TOXICITY:
Acute massive overdosage by oral ingestion of the ophthalmic ointment has not occurred. However, the rapid deamination to arabinosylhypoxanthine should preclude any difficulty. The oral LD50 for vidarabine is greater than 5020 mg/kg in mice and rats. No untoward effects should result from ingestion of the entire contents of the tube. Overdosage by ocular instillation is unlikely because any excess should be quickly expelled from the conjunctival sac.

AFECTED ORGANISMS:
Human Herpes Virus